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Effects of letrozole on breast cancer micro-metastatic tumor growth in bone and lung in mice inoculated with murine 4T1 cells

Abstract

Breast cancer (BC) is the leading cancer in women worldwide. Metastasis occurs in stage IV BC with bone and lung being common metastatic sites. Here we evaluate the effects of the aromatase inhibitor letrozole on BC micro-metastatic tumor growth in bone and lung metastasis in intact and ovariectomized (OVX) mice with murine estrogen receptor negative (ER−) BC cells inoculated in tibia. Forty-eight BALB/c mice were randomly assigned to one of four groups: OVX, OVX + Letrozole, Intact, and Intact + Letrozole, and injected with 4T1 cells intra-tibially. Letrozole was subcutaneously injected daily for 23 days at a dose of 1.75 µg/g body weight. Tumor progression was monitored by bioluminescence imaging (BLI). Following necropsy, inoculated tibiae were scanned via µCT and bone response to tumor was scored from 0 (no ectopic mineralization/osteolysis) to 5 (extensive ectopic mineralization/osteolysis). OVX mice had higher tibial pathology scores indicative of more extensive bone destruction than intact mice, irrespective of letrozole treatment. Letrozole decreased serum estradiol levels and reduced lung surface tumor numbers in intact animals. Furthermore, mice receiving letrozole had significantly fewer tumor colonies and fewer proliferative cells in the lung than OVX and intact controls based on H&E and Ki-67 staining, respectively. In conclusion, BC-inoculated OVX animals had higher tibia pathology scores than BC-inoculated intact animals and letrozole reduced BC metastases to lungs. These findings suggest that, by lowering systemic estrogen level and/or by interacting with the host organ, the aromatase inhibitor letrozole has the potential to reduce ER− BC metastasis to lung.

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Abbreviations

BC:

Breast cancer

BLI:

Bioluminescence imaging

E2:

Estradiol

ER:

Estrogen receptor

ER+:

Estrogen receptor positive

ER−:

Estrogen receptor negative

H&E:

Hematoxylin & eosin

HI-FBS:

Heat inactivated-fetal bovine serum

IMEM:

Iscove’s modified essential medium

µCT:

Micro computed tomography

MCF-7:

Michigan cancer foundation-7

OVX:

Ovariectomized

PBS:

Phosphate buffered saline

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Acknowledgments

This project was supported by Grant Number W81XWH-09-1-0689; BC085882 (WGH) from DOD Breast Cancer Research Program from U.S. Army Medical Research and Materiel Command and P50AT006268 (WGH) from the National Center for Complementary and Integrative Health (NCCIH), the Office of Dietary Supplements (ODS) and the National Cancer Institute (NCI). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NCCIH, ODS, NCI, or the National Institutes of Health.

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Correspondence to William G. Helferich.

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Wang, W., Belosay, A., Yang, X. et al. Effects of letrozole on breast cancer micro-metastatic tumor growth in bone and lung in mice inoculated with murine 4T1 cells. Clin Exp Metastasis 33, 475–485 (2016). https://doi.org/10.1007/s10585-016-9792-z

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Keywords

  • Aromatase inhibitor letrozole
  • Bioluminescence imaging
  • Breast cancer
  • Lung metastases
  • µCT
  • Micro-metastatic bone tumor
  • Ovariectomy